Either solution would be crazy awesome! The 5770's are a great deal though. Having two cards will take more power. Make sure you have at least 750Watts and more, if you have several hard drives and disc drives.
right- the 2x5770's are going to perform pretty close to a 5870 in games that work properly with crossfire (most newer games will work pretty well), but could drop you to basically just 1x5770 for games that don't.
Personally, I would probably go for a single 5850 just because it gives you more power with less hassle, and it also gives you an upgrade path for the future if you feel the need for it (adding a second one later)
Edit- you probably don't need 750W for 2x5770 either- 2x5770 actually uses about the same amount of power as a 5870, or just a little more, so really you'll be fine with the same size PSU you'd get for a 5870. You'd be fine with a good 500-600W PSU.
i have run both. had a 5850 with my 955 black ed. and now have dual 5770's with the 955 phenom II. gotta say the power consumption is nearly the same @ just over 420 watts at load from the wall socket,and all my benchmarks are higher with the crossfire set up. but actual game play is the same. both will max out almost anything you play. doing this on a 600 wat psu. 5770's are pretty efficient for the power they put out. and they scale great in most games, but not all.